Method for manufacturing semiconductor laser device

ABSTRACT

A semiconductor thin film including a well layer is laminated on a semiconductor substrate, the semiconductor substrate and the semiconductor thin film is cleaved, and a cleavage plane of the semiconductor substrate and the semiconductor thin film, which is obtained by the cleaving, is exposed to an atmosphere produced by decomposition of a gas containing N-atoms under the presence of a heated catalytic substance, thereby a surface layer of the cleavage plane is removed and a nitride layer is formed on the surface. Subsequently, a dielectric film is formed on the cleavage plane. According to the above technique, a natural oxide film formed on the cleavage plane can be removed and also a protective film can be formed by using a catalytic CVD apparatus.

TECHNICAL FIELD

The present invention relates to a method of manufacturing a high-powersemiconductor laser device with long-term reliability.

BACKGROUND ART

A semiconductor laser has been used for apparatuses in various fieldssuch as information communications, printings, processing, medicalapplications, or the like. It is necessary to improve power andreliability of the semiconductor laser as a light source, so as toimprove the performance of these apparatuses.

In general, the semiconductor laser has a structure in which an activelayer is sandwiched between a p-type cladding layer and an n-typecladding layer. Then, a substrate having the layers laminated thereon iscleaved and laser light is generated by applying a current to the activelayer using the cleavage plane as a resonator plane. Then, one of twocleavage planes serving as resonator planes, becomes a light outputtingpart. Further, the two cleavage planes are coated with a dielectric filmfor controlling the reflectance or suppressing deterioration with timecaused by chemical reaction on a cleavage surface.

When cleaving is carried out in general air atmosphere, a natural oxidefilm is formed on the cleavage surface. Taking GaAs compound as anexample, high-density surface levels, which is mainly caused by oxygenbinding of Ga and As, are present in the natural oxide film on thecleavage plane. Then, emitted light is absorbed by the natural oxidefilm as a non-radiative recombination center. Due to the lightabsorption, heat is generated in the vicinity of the cleavage plane, anda forbidden bandwidth of the active region is decreased, resulting infurther increasing light absorption. Consequently, the cleavage plane ismelted away, causing deterioration of the laser output considerably.Therefore, to achieve a high-power semiconductor laser with highreliability, it is necessary to preclude the formation of a naturaloxide film formed on the cleavage plane, particularly.

Conventionally, to prevent the natural oxide film from being formed, thefollowing processes are accomplished. That is, after cleaving is carriedout in high vacuum, a protection layer is formed without exposing thecleavage plane to the air before forming a natural oxide film is formed,or after cleaving is carried out in an atmospheric air, the naturaloxide film formed on the cleavage plane is removed by an electron beamheating, a laser irradiation, or plasma exposure using an inert gas soas to form a protection film. In addition, another method is alsoaccomplished. That is, after placing the cleavage plane into a vacuumapparatus, the cleavage plane is exposed to a halogen gas at 400° C. orhigher. Then, an oxide layer is removed by thermochemical reaction, anda compound semiconductor layer and the like is formed thereon.

However, the above mentioned cleavage operation in high vacuum isrequired for extremely high vacuum level depending on the process time,resulting in requiring high cost or strict control of apparatuses.

Further, according to the method of forming a protection film byremoving a natural oxide film by means of an electron beam heating, alaser irradiation, or plasma exposure using an inert gas, the naturaloxide film or surface contaminants is removed by a physical method,mainly. Therefore, there is a concern that defects are introduced in asurface layer in addition to the removal of these. Using the abovemethods, in particular, oxygen binding of Ga and As can be removed,however, the introduced defects function as a recombination center.Consequently, it is necessary to perform precise control of processingconditions or the like for an improvement of these methods.

Further, according to the method of thermochemical reaction with ahalogen gas, since it is necessary to heat the halogen gas to 400° C. orhigher, an electrode cannot be formed before the cleavage operation.Instead, an electrode is formed after forming a protection film for theresonator plane which is formed by cleaving. Consequently, there existsa problem in that processes become inconvenient and complicated.

DISCLOSURE OF INVENTION

The invention is proposed to solve the above problems. According to theinvention, a natural oxide film formed on a cleavage plane is removedand also a protection film is formed by using a catalytic Chemical VaporDeposition (CVD) apparatus.

Namely, the invention provides a method of manufacturing a semiconductorlaser comprising the steps of:

laminating a semiconductor thin film comprising a well layer on asemiconductor substrate;

cleaving the semiconductor substrate and the semiconductor thin film;

exposing a cleavage plane of the semiconductor substrate andsemiconductor thin film obtained by cleaving to an atmosphere producedby decomposition of a gas containing N atoms, under the presence ofheated catalytic substances, thereby removing the surface layer of thecleavage plane and forming a nitride layer on the surface; and

subsequently forming a dielectric film on the cleavage plane.

According to the invention, even if the resonator plane of thesemiconductor laser is formed by cleaving in the air, a surface layermade of a natural oxide film formed on the cleavage plane is exposed ina vacuum apparatus to a gas containing N-atoms, which are changed intoradical in the catalytic CVD apparatus. By doing this, etching removalcan be carried out at a low substrate temperature with extremely lowlevel of damage of the semiconductor thin film, and at the same time anitride layer, which has excellent chemical stability, can be formed. Asthe gas containing N-atoms, ammonia (NH₃), hydrazine (NH₂NH₂), or thelike can be used. Since the nitride layer has a wide band gap andterminates and decreases defects, it is very preferable material in viewof junction between a semiconductor and a dielectric film. In general,when GaAs is used in III-V group semiconductor laser, however, a GaNlayer is formed therein.

Subsequently, by forming a dielectric film on the cleavage plane, thedielectric film is formed on the face from which the natural oxide filmis removed. Because of this, it is possible to prevent temperature fromincreasing due to light absorption and to prevent the cleavage planefrom melting when emitting laser light. Then, since the nitride layerformed on the cleavage plane, from which the natural oxide film isremoved, has excellent chemical stability, reoxidation will not occureven if the cleavage plane is exposed to the air. Therefore, between thestep of exposing the cleavage plane to the atmosphere produced bydecomposition of a gas containing N-atoms using the catalytic CVDapparatus and the step of forming the dielectric film, it is allowed toexpose the semiconductor substrate to the air.

Further, in comparison with the case where plasma process such assputtering is used for forming the dielectric film, the method in whichafter eliminating the natural oxide film and forming the nitride film bythe catalytic CVD apparatus then forming the silicon nitride film bymeans of the catalytic CVD apparatus is preferable because plasma damagecaused by ion impacts on the cleavage plane can be eliminated. That is,after removing the natural oxide film and forming the nitride film bymeans of the catalytic CVD apparatus, the silicon nitride film issubsequently formed by using the same catalytic CVD apparatus. Thesilicon nitride film is formed by exposing the cleavage plane to anatmosphere produced by decomposition of a gas containing N and Si, or agas containing N and a gas containing Si, under the presence of heatedcatalytic substances.

In the invention, it is preferable that a well layer of thesemiconductor laser manufactured by the above steps, is made of acomposition of any elements selected from In, Al, Ga, P and As. Theseelements will form a chemically stable nitride film.

BRIEF DESCRIPTION OF DRAWINGS

Other and further objects, features, and advantages of the inventionwill be more explicit from the following detailed description taken withreference to the drawings wherein:

FIG. 1 is a diagram showing relationship between a holder and a cleavageplane according to an example;

FIG. 2 is a schematic view showing a catalytic CVD apparatus andsurroundings thereof utilized in the example;

FIG. 3 is a schematic view showing a semiconductor laser chip obtainedfrom the example;

FIG. 4 is an output plot of a semiconductor laser obtained from theexample;

FIG. 5 is an output plot of a semiconductor laser obtained from thecomparative example;

FIG. 6 shows X-ray photoelectron spectroscopy (XPS) plots regardingsamples such as As3d and Ga3d obtained from the embodiment and thecomparative example;

FIG. 7 shows an XPS plot regarding a sample such as N1s obtained fromthe example; and

FIG. 8 shows XPS plots regarding a sample such as A12p obtained from theexample and the comparative example.

BEST MODE FOR CARRYING OUT THE INVENTION

The invention will be described in detail. A semiconductor lasercomprises a semiconductor substrate, an active region formed thereon, atleast a pair of cladding layers which sandwiches the active region, andp-side and n-side electrodes formed on the upper and lower surfaces, andthe laser is formed on a wafer. Then, the wafer is cleaved into a barshape in the air or nitrogen so as to have a desired resonator length,resulting in forming a semiconductor laser bar. The semiconductor laserbar is put into a vacuum apparatus using a holder so that the cleavageplane serving as a resonator plane is exposed to an atmosphere producedby decomposition of a gas containing N-atoms using a catalytic CVDapparatus. The catalytic CVD apparatus is used for the method ofperforming surface treatment or film formation, in which filament suchas tungsten, which is a catalytic substance, is heated in a vacuumvessel and sprayed with a raw material gas, thereby generating a radicalof the raw material gas by thermal contact decomposition with the use ofcatalytic action. The method is described in more detail in HidekiMatsumura, Jpn. J. Appl. Phys. 37, 3175 (1998), for example.

First, air is exhausted from the vacuum apparatus in which the holder tobe stacked with a semiconductor laser chip is placed, by using a vacuumpump, to form a vacuum atmosphere at 1×10⁻⁴ Pa or lower. Subsequently,NH₃ gas is introduced therein. In addition, the gas may be diluted withH₂ to control the etching speed of the natural oxide film. The flow rateor pressure of the gas introduced is varied depending on the pumpperformance or conditions of the apparatus. In particular, the achievedamount of the radical obtained by decomposition of a gas containing Natoms varies depending on the distance between the filament and thesubstrate and the pressure. Because of this, the substrate surfacetemperature and the processing time are also varied, so thatoptimization is necessary in respective apparatuses and in respectivecases. For example, when the distance between the filament and thesubstrate is 60 mm, it is preferable that the pressure is approximately0.75 Pa.

Subsequently, the filament is heated by DC power or the like. Whentungsten is used as the filament, the filament surface temperature isrequired to be the temperature or higher, at which a gas containingN-atoms can be decomposed. For instance, NH₃ gas will be decomposed at1000° C. While decomposed and generated radical species or decompositionefficiency is varied depending on the filament temperature, heatradiation from heated filament increases the substrate temperature.Since the amount of the temperature increase further depends on thepressure and the distance between the filament and the substrate as wellas the filament temperature, the filament temperature should beoptimized in consideration of the above matters. When the substratetemperature rises, the etching speed is increased. In the case of aradical produced by decomposition of NH₃ gas, the cleavage surface tendsto be coarse. In general, in view of preventing an increase of thesubstrate temperature due to heat radiation, the temperature of 1400° C.or lower is desired as the filament temperature. Further, to prevent anincrease of the substrate temperature, it is effective to coolsurroundings of the substrate by means of water cooling.

After introducing gas, the filament temperature is increased-to be thetemperature at which a gas containing N atoms can be decomposed. Then,the cleavage plane is exposed to a radical produced by decomposition ofthe gas containing N atoms, whereby etching of the cleavage plane can beperformed. According to the method, heat contact decomposition usingcatalytic action is adopted instead of the decomposition utilizing highfrequency electric field, so that damages which accompany ion generationor defects generated on the cleavage surface due to collision ofaccelerated ions, extremely become small. Further, nitrogenation of thesurface occurs at the same time. The formation of GaN on a surface layerhas an effect of terminating and decreasing the defects. In addition tothis, since GaN has a wide band gap with respect to an active regioncomposed of a composition of any elements selected from In, Al, Ga, P,and As, the formation of GaN is preferable in view of junction between asemiconductor and a dielectric film.

Furthermore, since GaN has excellent chemical stability, once GaN isformed, reoxidation will not occur even if the cleavage plane is exposedto the air. Therefore, it is possible to transport it in the air at thetime of subsequently forming a dielectric film. Consequently, stepsbecome simple. The process time varies depending upon adopted apparatusas mentioned above, however, it can be optimized by the check ofroughness on the cleavage surface with AFM (intermolecular forcemicroscope) or the binding state of oxygen and nitrogen, which arecomposition elements of the active region, with XPS. As an example, itis preferable to apply the method disclosed in A. Izumi et al./ThinSolid Films 343-344(1999)528-531.

Further, the portion except for the cleavage plane of the semiconductorlaser bar, namely, upper and lower faces of the bar are coated withmetal electrode while etching process. Etching rate of gold, gold alloy,platinum, or the like, which is commonly used for an electrode of thesemiconductor laser, is extremely slow in comparison to that of acompound semiconductor. Because of this, when exposing the portionexcept for the cleavage plane to a radical produced by decomposition ofa gas containing N-atoms, there is no damaged portion on the portionexcept for the cleavage plane within the range of time required forremoving an oxide layer on the cleavage surface. Further, whensemiconductor laser bars are laminated inside of the holder in order toexpose the cleavage edge face of the semiconductor laser through thewindow portion of the holder, the portion except for the cleavage planeof the semiconductor laser bar is not exposed to the radical produced bydecomposition of a gas containing N-atoms. Also, it is possible toprevent adhesion of the film to the portion except for the cleavageplane in a later film formation step.

By means of the catalytic CVD apparatus, the oxide layer formed on thecleavage surface is removed by etching using a radical produced bydecomposition of a gas containing N-atoms and a nitride layer is formed,and thereafter a dielectric film is formed. Herein, the dielectric filmis formed so as to control the reflectance of the cleavage plane,mainly.

Sputtering, CVD film formation, or the like can be used to form adielectric film. As the dielectric film, an aluminum oxide film, analuminum nitride film, a silicon film, a silicon oxide film, a siliconnitride film, a titanium oxide, or a lamination film thereof ispreferable, and in particular, in order to suppress reoxidation causedby the dielectric film formation process on the cleavage surface, anon-oxide film is more suitable among the above mentioned films for theprotection film which comes in contact with the cleavage plane.

Then, the cleavage plane is exposed to an atmosphere of a radicalproduced by decomposition of a gas containing N-atoms by using thecatalytic CVD apparatus, whereby a surface layer such as a natural oxidefilm formed on the cleavage plane is removed and at the same time, anitride layer is formed on the cleavage surface. Thereafter, anadditional passivation film maybe formed before forming the dielectricfilm for controlling the reflectance to enhance the passivasion effect.

When considering the above matter, in comparison with the case where thedielectric film is formed by plasma process such as sputtering, thefollowing process is preferable because it can prevent the plasma damagedue to ion impact onto the cleavage plane at the time of forming adielectric film. That is, after exposing to an atmosphere of a radicalproduced by decomposition of the gas containing N-atoms, a siliconnitride film is subsequently formed by using the same catalytic CVDapparatus. Further, since the silicon nitride film formed by thecatalytic CVD apparatus, has a low film stress in the order of 10⁹dyn/cm², it is preferable in the point that film peeling with time willrarely occur in comparison with the silicon nitride film formed by usualsputtering process. The silicon nitride film can be formed by supplyinga gas containing N-atoms and SiH₄ gas in the catalytic CVD apparatusused for generating a radical produced by decomposition of the gascontaining N-atoms to keep the filament temperature to be not lower thanthe temperature at which filament does not form silicide and not higherthan the temperature at which vapor pressure of filament does not causea problem. For instance, in the case of using tungsten as filament, thetemperature at which film can be formed is within the range of from1600° C. to 1900° C. As the flow rate of the gas containing N-atoms andthe SiH₄ gas, optimum value which makes film stress the lowest value,may be used. Further, when thermal damage, which is caused on thecleavage edge face due to an increase of the filament temperature,becomes a problem, by reducing the film formation time, the siliconnitride film is formed to a thickness serving as a protection layer fromplasma damage, for example, to a thickness of about 2 to 10 nm.Subsequently, a dielectric film having a desired reflectance may beformed by another process such as sputtering.

The semiconductor laser device according to the invention is not limitedto its epitaxial structure or its composition, and can be widelyapplicable to any structure. To achieve higher power, the semiconductorlaser device may have a structure as cladding layers in which a firstcladding layer and a second cladding layer having a lower refractiveindex and a wider band gap than the first cladding layer, are providedviewed from the active region side, or completely separated confinementstructure in which carrier blocking layers, waveguide layers, andcladding layers are provided on both sides of an active region, andwhich satisfies the relationship that carrier blocking layers have alower refractive index than waveguide layers and cladding layers have alower refractive index than active regions (See U.S. Pat. No. 005764668Afor detail). Further, as the composition of an active region used for adevice, GaAs, AlGaAs, InGaAs, or InGaAsP may be selected depending onthe oscillation wavelength. Needless to say, another composition may beutilized and in particular, it is preferable to use composition havingsmaller band gap than GaN.

EXAMPLE

The semiconductor laser has completely separated confinement structurein which a carrier blocking layer is interposed between an active regionand a waveguide layer, and has a stripe width of 8 μm. Then, thesemiconductor laser is designed to oscillates in a single mode in thewavelength of 860 nm range comprising a cladding layer made of AlGaAs, awaveguide layer made of AlGaAs, and an active region formed by heterojunction between AlGaAs and GaAs. A wafer to be formed with thesemiconductor laser is cleaved into a bar-shape in the air so as to forma resonator length of 1.4 mm. Then, some of the semiconductor laser barsobtained by the cleaving, are placed in a holder. FIG. 1 shows the abovestate, more specifically, shows the plane which is exposed to a radicalproduced by decomposition of NH₃ gas by the catalytic CVD apparatus. Inthe holder 1, two semiconductor laser bars 2 a and 2 b and a dummy bar 3are stacked on top of each other in layers so as to expose cleavageplanes of the semiconductor laser bars 2 a and 2 b and the edge face ofthe dummy bar 3, which are formed on the same plane, to the windowprovided in the holder 1. Then, the holder 1 is put into the catalyticCVD apparatus. The catalytic CVD apparatus having the structure as shownin FIG. 2 is used herein. The holder 1 in which the semiconductor laserbars are stacked, is placed on a water-cooled board 5.

After the vacuum apparatus 12 is evacuated to ultimate vacuum of 3×10⁻⁵Pa by a rotary pump 7 and a turbo molecular pump 6, NH₃ gas of 50 sccmis introduced through a flow meter 8 and the pressure of the vacuumapparatus is maintained to be 0.75 Pa by a pressure control bulb 10.Then, the surface temperature of a tungsten filament 4, which ismonitored with an infrared radiation thermometer 9, is heated to 1200°C. with a DC supply 11. By opening a shutter 13, the cleavage plane ofthe semiconductor laser bar, which is exposed to the window of theholder 1, is exposed to a radical produced by decomposition of NH₃ gasfor three minutes. Then, after the treatment for three minutes, theshutter 13 is closed, heating of the filament is stopped, the flow rateof NH₃ gas is increased to be 60 sccm, and subsequently, SiH₄ gas of 1sccm is introduced through a flow meter 14, and filament is again heatedto 1800° C. In the state, the shutter 13 is opened and the cleavageplane of the semiconductor laser bar, which is exposed to the window ofthe holder 1, is exposed to radicals produced by decomposition of NH₃gas and SiH₄ gas for two minutes, thereby the silicon nitride film isformed. At this time, the thickness of the film deposition is about 4nm, which is based on the film deposition speed as conditions studied inadvance. After forming the silicon nitride film, heating of the filamentis stopped, introduction of SiH₄ gas and NH₃ gas is stopped, and thengas is exhausted with a vacuum pump. Subsequently, the holder to bestacked with the semiconductor laser bars is taken out from the vacuumvessel and is turned upside down. Then, the same treatment is performedon the opposite side of the cleavage plane. The holder to be stackedwith semiconductor laser bars, in which both faces of the cleavage planeare treated, is moved to another vacuum apparatus, an anti-reflectivity(AR) coating with reflectance of 2% is applied on both faces of thecleavage plane by sputtering film deposition of an aluminum oxide.Further, high reflectivity (HR) coating with reflectance of 97% isapplied on only one face of the cleavage plane by sputtering filmdeposition of a Si/SiO₂ multilayer film.

These semiconductor laser bars are cut to form a chip-shape, obtaining asemiconductor laser chip as shown in FIG. 3. On a light outputting endsurface which outputs a laser light, a silicon nitride film 21 and alamination film 24 made of Al₂O₃ 22 are formed. On the opposite endface, the second lamination film 25 provided with a Si/SiO₂ multilayerfilm 23, is formed. After mounting the semiconductor laser chip on amounting, to examine the intensity of the end face portion of lightemission, maximum light output is examined by applying CW current at 25°C. As a result, catastrophic optical damage (COD) level shows high valueof 1.4 W as shown in FIG. 4.

Further, the surface of the AlGaAs layer of the sample, in whichepitaxial growths of an AlGaAs layer having 2 μm thickness is performedon a GaAs substrate, is treated by a radical produced by decompositionof NH₃ following the above mentioned procedure. Then, the surface isexamined by XPS to check the binding state of surface elements. As aresult, the binding caused by an oxide is not observed on As3d as shownin FIG. 6 and as shown in FIG. 7, N1s peak can be observed on thesample. Further, as shown in FIG. 8, high energy shift can be observedwith respect to Al2p. According to these results, it is confirmed thatelements for oxygen binding are decreased and a nitride layer containingAlGaN as a main component is formed on the surface of AlGaAs.

Comparative Example 1

The semiconductor laser device, which is the same as that of theexample, is cleaved into a bar-shape in the air and stacked in theholder. Then, the holder is put into the sputtering apparatus, and ARcoating with reflectance of 2% is applied on both faces of the cleavageplane by forming aluminum nitride and subsequently an aluminum oxide bysputtering. Further, HR coating with reflectance of 97% is applied ononly one face of the cleavage plane by forming a Si/SiO₂ multilayer filmby sputtering. After thus formed semiconductor laser bars are cut into achip-shape and mounted on a mounting, maximum light output is examinedin the same manner as the example. As a result, catastrophic opticaldamage (COD) level shows about 1.2 W as shown in FIG. 5.

Moreover, in the same way as the example, to check binding state ofsurface elements regarding the sample in which an AlGaAs layer of 2 μmin thickness is formed on a GaAs substrate, the surface of the AlGaAslayer is examined by XPS. As a result, only the binding caused by anoxide is observed on Al, Ga, and As as shown in FIG. 6 and FIG. 8.

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and the rangeof equivalency of the claims are therefore intended to be embracedtherein.

EFFECT OF THE INVENTION

As described above, according to the invention, a high-powersemiconductor laser device with high reliability can be achieved bytreatment of the light emitting end face using relatively simple method.According to the method, a resonator plane of a semiconductor laser isformed by cleaving in the air and then put into a vacuum apparatus.Then, a natural oxide film formed on the cleavage plane is exposed to aradical gas containing N-atoms produced in the catalytic CVD apparatus,thereby removing by etching and forming a nitride layer at the sametime. Subsequently, a dielectric film is formed on the surface.

What is claimed is:
 1. A method of manufacturing a semiconductor lasercomprising: laminating a semiconductor thin film comprising an activelayer on a semiconductor substrate; cleaving the semiconductor substrateand the semiconductor thin film; exposing a cleavage plane of thesemiconductor substrate and semiconductor thin film obtained by cleavingto an atmosphere produced by decomposition of a gas containing N atoms,under the presence of heated catalytic substances, thereby removing thesurface layer of the cleavage plane and forming a nitride layer on thesurface; and subsequently forming a dielectric film on the cleavageplane.
 2. The method of manufacturing a semiconductor laser of claim 1,wherein the dielectric film is formed by exposing the cleavage plane toan atmosphere produced by decomposition of a gas containing N and Si, ora gas containing N and a gas containing Si, under the presence of heatedcatalytic substances.
 3. The method of manufacturing a semiconductorlaser of claim 2, wherein the active layer is made of a composition ofany elements selected from In, Al, Ga, P and As.
 4. The method ofmanufacturing a semiconductor laser of claim 1, wherein the active layeris made of a composition of any elements selected from In, Al, Ga, P andAs.